Method and apparatus for recording digital images on photosensitive material

ABSTRACT

The present invention is directed to a method and apparatus for exposing photosensitive material to form high quality continuous tone and/or color images thereon. The preferred apparatus includes an imaging head comprised of a plurality of red light sources, a plurality of green light sources, and a plurality of blue light sources. The light produced by said green light sources is passed through a first filter having a narrow spectral transmission characteristic in the green spectral range. Similarly, the light produced by said blue light sources is passed through a second filter having a narrow spectral transmission characteristic in the blue spectral range.

RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.09/303,258 filed on Apr. 30, 1999 now abandoned which claims prioritybased on provisional application 60/083,975 filed May 1, 1998.

FIELD OF THE INVENTION

This invention relates generally to a method and apparatus for producinghigh quality continuous tone and/or color images on photosensitivematerial (i.e., photographic paper or film) from information provided indigital form.

DESCRIPTION OF PRIOR ART

In the field of photographic digital printers and image setters, the useof multiple light sources to create individual pixels is well known.U.S. Pat. No. 3,988,742 describes using LEDs and fiber optic lightguides to deliver the light to the photosensitive material. Applicationsof this technology have included type setting, and the generation oflithographic films for printing. In these applications, the light outputof LEDs is coupled into the input end of the fiber optic tubes. Theoutput ends of the fiber optic tubes are arranged in a linear array. Asphotosensitive material is passed by the linear array of fiber optictubes, the LED's are illuminated in such sequence as to cause theformation of indicia or images on the photosensitive material. Thisprocess is described in U.S. Pat. Nos. 3,832,488 and 4,000,495 and5,093,682. The use of fiber optic tubes of both square and round crosssections is known.

In such applications as described above, the precision assembly of theoutput ends of the fiber optics is important. Poor alignment, or unevenspacing of the output ends of the fibers cause distortions in the imagesbeing generated. U.S. Pat. Nos. 4,364,064 and 4,389,655 describe devicesfor precisely positioning the fiber optic tubes. U.S. Pat. No. 4,590,492describes a method for masking the ends of the fiber optic tubes toprovide more precise alignment of the light sources exposing thephotosensitive material.

Prior art systems have typically been used to form lithographic imagesand indicia for typesetting and printing consisting solely of white andblack areas without intermediate tones. Due to the lack of intermediatetonal detail, such systems are tolerant of some imprecision in thequality and quantity of light delivered to the photosensitive material.More particularly, they are typically tolerant of imperfect pixel topixel alignment because of slight pixel blooming which occurs as aconsequence of using exposure levels high enough to saturate thephotosensitive material.

Continuous tone images, e.g., images which are predominately composed ofmiddle tones, whether colors or gray tones, require significantprecision in pixel formation and alignment. Misalignment of one pixelrelative to its neighbors will cause unwanted lines or other artifactsto appear in a continuous tone image.

SUMMARY OF THE INVENTION

The present invention is directed to a method and apparatus for exposingphotosensitive material to form high quality continuous tone, colorimages thereon.

Embodiments of the invention are typically comprised of an imaging (orprint) head comprised of multiple pixel image generators, e.g., lightsources. The head is preferably mounted for linear movement in a first,i.e., scanning direction, across the width of a web of photosensitivematerial. The photosensitive material is mounted for linear movement ina second direction perpendicular to said first direction to enablesuccessive scan strips (i.e., groups of scan lines) to be imaged ontosaid photosensitive material. The portion of the web to be printed canbe referred to as an “image field” and can be considered to consist of arectangular matrix of “rows” extending in the scan direction across theweb width and “columns” extending perpendicular to the rows, i.e.,longitudinally along the web. The head can be stepped or movedcontinuously in the scan direction with the multiple light sources beingselectively enabled to expose an image onto the photosensitive material.

A system in accordance with the invention produces a field of“interpolated pixels”, each interpolated pixel being formed by theoverlap between adjacent pixel images. More particularly, aninterpolated pixel in accordance with the invention can be formed by theoverlap between adjacent pixel images displaced in the scan direction,e.g., horizontal, and/or by the overlap between adjacent pixel imagesdisplaced in the longitudinal direction, e.g., vertical.

It is an object of this invention to provide a method and apparatus forprecisely delivering light to photosensitive material to allow theprinting of extremely high quality continuous tone images from digitalinformation.

It is a further object of this invention to provide a method andapparatus for blending the pixels of an image presented in digital form,so as to increase the apparent resolution and sharpness of the resultingprinted image.

It is a further object of this invention to provide a low cost imaginghead capable of precisely delivering light to photosensitive materialwith the precision required to allow the printing of extremely highquality continuous tone images.

In accordance with a preferred embodiment of the invention, the imaginghead is comprised of square or rectangular pixel image generators, e.g.,fiber optic tubes, mounted to form a rectangular array . The pixel imagegenerators are inclined at an angle of 45 degrees to the scan direction.As the print head scans, each fiber optic tube can expose a pixel imageonto the photosensitive material. The exposure levels of the pixelimages are preferably specified in a digital file representing an imageto be printed. As a result of being inclined at 45 degrees, the pixelimages exposed onto the photosensitive material are diamond shaped. Eachpixel image overlaps its neighbor by substantially 50% of the center tocenter distance between pixel images. The pattern resulting from theoverlapping of the pixels images generates geometrically interpolatedpixel areas with each such area being about 25% of the original pixelimage area. Further, the shape of each pixel relative to the scandirection causes the exposure in the area between adjacent scan lines toremain consistent and to cause superior blending of each pixel imagewith its neighboring pixel images.

The generation of high quality continuous tone images requires theprecise blending of the pixels imaged on the photosensitive material.Precise blending of the pixels requires that the pixels themselves be ofuniform color and intensity. Fiber optic tubes operate by the principleof “total internal reflection” of the light waves introduced into thefiber optic tube. The symmetric nature of round fiber optic tubes issuch that the image of the light source at the input end of the tube isdelivered, more or less intact, to the output end of the tube. Thenature of LEDs and other light sources is that the light emittingelement does not emit perfectly uniform illumination. Consequently, theimage of the LED die will propagate down a round fiber optic tube and bedelivered to the end. The non-uniform nature of the image of the lightelement will cause the pixels to blend poorly with one another. In afiber optic tube of square or rectangular cross section, the image isscrambled by the successive reflections off the orthogonal walls of thetube. The principles of “total internal reflection” causes most all ofthe light energy which enters the fiber optic tube to be delivered tothe output, but with the image scrambled to such an extent as to makethe light output from the fiber optic tube substantially uniform. Thisuniformity is desirable to achieve the highest quality of continuoustone images.

As described above, the generation of high quality continuous toneimages requires extreme precision in the placement and uniformity ofillumination of the pixels comprising the image. Imprecision in eitherof these will result in artifacts or lines appearing in the printedimage. In the printing method as described herein, a plurality ofindependently excitable light sources is employed to provide theillumination for a matching number of pixels. When printing onto colorphotosensitive materials, extreme precision is required in the matchingof the spectral output characteristics of the multiple light sources. Ifthe light sources are not of precisely the same spectralcharacteristics, artifacts or lines will appear in the printed image.Light sources of different spectral characteristics will exposedifferent layers of the photosensitive material with differing efficacy.It is possible to adjust the intensity of light sources to be equallyeffective in exposure at a particular color or shade. However, if thespectral characteristics of the light sources are not precisely matchedto one another, they will not be equally effective at exposing adifferent color. The result will be that artifacts or lines appear insome colors of the printed image, but not others.

In a preferred embodiment of the invention, the individual light sourcesare matched in spectral output with the use of a narrow pass band filterfor each color. The narrow pass nature of the filter restrains theexposing energy of each LED to a narrow wavelength range within whichthe photosensitive material will have uniform color response. The filteris fabricated in such a way as to cover all of the pixels of a givencolor with the same filter. In the embodiment of the invention actuallyconstructed by the inventor, filters of two independent wavelengths werefabricated on the same substrate and placed over the ends of the fibers.

The print head can be imaged onto the photosensitive material either byintimate contact or via a lens system. In one implemented embodiment ofthe invention, the print head is comprised of three columns of fiberoptic tube ends, each column containing 32 tube ends. The head scansacross a 30 inch width of photosensitive material and exposes a strip ofapproximately 0.100 inches during each scan. After each scan, thematerial is advanced in the longitudinal direction by the height of theexposed strip area. The head then successively scans across thephotosensitive material exposing additional strip areas to fully coverthe image field. In an alternative embodiment of the invention, the headcould be the full width of the material obviating the need for the headto scan across the width of the material.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic isometric illustration of a print head inaccordance with the invention depicting a rectangular array of squarefiber optic tube ends;

FIG. 2 is a schematic isometric illustration of a portion of a digitalprinter in accordance with the invention showing a drum for transportinga photosensitive material web and a print head mounted for linearmovement to scan across the width of the web;

FIG. 3 is a schematic illustration depicting a prior art scheme forenabling a print head comprised of a column of square light sources toscan across a web of photosensitive material to expose pixel imagesthereon;

FIG. 4A is a schematic illustration depicting a preferred system inaccordance with the invention for exposing pixel images onto thephotosensitive material; and FIG. 4B schematically depicts analternative head configuration;

FIGS. 5 and 6 are schematic illustrations showing how longitudinallydisplaced pixel images overlay in the invention to form interpolatedpixels;

FIG. 7 is a schematic isometric illustration depicting a color matchingfilter for installation on the print head;

FIG. 8 depicts typical spectral characteristics of red, green and blueLED light sources;

FIG. 9 depicts a typical range of spectral characteristics of a group ofLED light sources; and

FIG. 10 shows the spectral transmission characteristics of the colormatching filter shown in FIG. 7.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 shows the output end of an imaging or print head 1 in accordancewith a preferred embodiment of the invention. The head is comprised of aplurality of pixel image generators, e.g., light sources defined byfiber optic tubes. The tubes are preferably of substantially square orrectangular cross section and are arranged in columns 5, 6, 7. The fiberoptic tubes are selected or manufactured to be of precise crosssectional dimension. With the aid of an assembly fixture, the fiberoptic tubes are clamped by frame 4 or bonded in place. Each column iscomprised of multiple fiber optic tube output ends which are assembledand positioned in precise alignment with one another. The columns may bearranged in contact, or spaced apart with precision spacers 8. In eitherconfiguration, each column contains the same number of fiber optic tubesand is precisely the same length. After assembly, the fiber optic tubeends are machined to a predetermined cross section and polished.Although the assembly as shown in FIG. 1 has a planar cross section,other shapes are possible. Specifically, curved cross sections arepossible which have the advantage of being able to contour to thesurface of a drum around which the photosensitive material is placed, orto correct for focus aberrations common to lenses. The input ends of thefiber optic tubes are connected (not shown) to independently excitablelight sources, preferably light emitting diodes (LEDs). The embodimentof the invention as shown uses fiber optic tubes to deliver the light tophotosensitive material and to define the shape of pixel images exposedthereon. An alternative embodiment of the invention can employsubstantially square LED dice arranged in a matrix and mounted directlyinto frame 4.

FIG. 2 shows the head 1 in place in one possible application of theinvention. In this application, the image of the output ends of thefiber optic tubes 3 is projected via a lens 14 onto a target surface,i.e., a web or sheet of photosensitive material 23, which is tensionedaround a drum 10. The print head assembly 15 and lens assembly 16 aremounted for linear lateral movement in a scan direction 18 parallel tothe axis of the drum 10 around which the photosensitive material 23 ispositioned. As the print head scans, encoder 24 indicates the columnarposition of the print head along the scan line. The photosensitivematerial 23 is moved longitudinally perpendicular to the scan direction18.

FIG. 3 shows a representation of a print head 37 which has beendescribed in the prior art. Light sources 38, identified as S₀ throughS_(n), are of square cross section, and expose pixel images representedin the grid 39. As the print head scans across a row of thephotosensitive material (defining an image field comprised of m columnsand n rows), a pixel image can be exposed at each columnar position P₀through P_(m). The exposures of the pixel images are enabled by nseparate enabling signals 46 E₀ through E_(n). The enabling signals aresynchronized with the encoder 24 indicated in FIG. 2.

FIG. 4A shows a representation of a preferred head 1 in accordance withthe invention. The print head is inclined at an angle of 45 degrees tothe scan direction. Light sources 76 S₀ through S_(n), define a diamondshaped cross section as a result of the square ends being inclined at 45degrees relative to the scan direction. Light sources 76 S₀ throughS_(n), expose pixel images 78 as represented in the grid 80. As theprint head 70 scans across the photosensitive material, the pixel imagescan be exposed at each position P₀ through P_(m). As a consequence ofbeing inclined at 45 degrees, the pixel images associated with a singlecolumnar position P are not exposed simultaneously. Instead, pixelP_(m),S₀ is exposed at the same time as pixels P_(m-1),S₁ andP_(m-2),S₂, etc. The exposures of the pixel images are enabled by nseparate enabling signals 82 E₀ through E_(n). The enabling signals aresynchronized with the encoder 24 indicated in FIG. 2. As the head scans,the pixel images in accordance with the invention overlap. That is, thepixel images of each scan row overlap with the pixel images of rowsabove and below by substantially 50% of the center to center distancebetween pixel images. Also, each pixel image along a row overlaps withpixel images horizontally displaced before and after by substantially50% of the center to center distance between pixel images.

Whereas the head in FIG. 4A scans laterally perpendicular to thelongitudinal direction of the photosensitive material 23, FIG. 4Bdepicts an alternative head configuration 83 in which the head can befixed. More particularly, FIG. 4B depicts a head extending across thewidth of the photosensitive material 23. The head 83 is comprised of atleast two rows of square pixel image generators (e.g., fiber optic tubeends) oriented at a 45 degree angle with respect to the lateral andlongitudinal directions. The head 83 is capable of imaging the samepixel image pattern as is depicted in FIG. 4A.

Referring to FIG. 5, as the photosensitive material is exposed by thepixel images, the exposure contribution from pixel image 26 to its scanline 28 is greatest in the center portion because the pixel 26 is widestalong that line. The exposure level decreases linearly with distancetransverse to the center line 30 of the pixel. The exposure leveldecreases to the point where it is zero at the center line 32 of theadjacent scan line 34. Correspondingly, the exposure contribution fromthe adjacent pixel image 36 to the raster line 28 decreases with thedistance from it's center line 32. It can be seen that at any point on aline between the center of the pixel image 26 and the center of theadjacent pixel image 36, the exposure level is comprised of a portion ofeach pixel image. Due to the shape of the pixel image, it can be seenthat the exposure level in the region between the center line 30 of thepixel image 26 and center line 32 of the adjacent pixel image 36 is alinear average of pixel 26 and the adjacent pixel 36.

FIG. 6 shows the pattern of diamond pixels 40 relative to a traditionalsquare pixel pattern 42 as represented in FIG. 3. It can be seen thatthe overlapping of the diamond pixels 40 generates diamond shapedinterpolated pixels 44. The interpolated pixels have an area equal toabout 25% of the original pixels 40. The side dimension of the diamondshaped interpolated pixels 44 (which in a constructed exemplaryembodiment is equal to 0.0047 inches) is 0.707 (=1/square root of 2)times the side dimension of the traditional square pixel 42. For a givendata input, these smaller interpolated pixels 44 cause an increasedvisual resolution with a corresponding increase in apparent sharpness inthe final image.

The interpolated pixels 44 create an effective blending because, inpart:

-   -   1) The head is in motion as it exposes pixel images which causes        the amount of light delivered to the photosensitive material to        be greatest where the pixel is widest in the scan direction, and    -   2) The photosensitive material is generally not sufficiently        resolute to resolve a sharp image of one diamond pixel.        Consequently the material diffuses adjacent pixels into each        other to some degree.

FIG. 8 shows the spectral characteristics of typical red 64, green 62and blue 60 LED light sources. Due to manufacturing variations common inLED technology, there can be considerable variation in the spectraloutput from similar LEDs. FIG. 9 shows a typical range of variation inspectral outputs in a small population of red, green and blue LED lightsources, respectively 64′, 62′ and 60′. FIG. 10 shows the narrow passspectral transmission characteristics 72, 74 of blue and green filtersdepicted in FIG. 7. By passing the energy of each of the LEDs in thepopulation of curves shown in FIG. 9, through a filter whose spectralresponse is shown in FIG. 10, the spectral output of each of the LEDs isconstrained to fall within the envelope of the filter. After filtering,all light sources will have substantially the same spectralcharacteristics and function substantially the same when exposing thedifferent colors of the photosensitive material.

FIG. 7 shows a preferred color matching filter 52 installed on the printhead 1. The color filter 52 is comprised of three sections: blue 54,green 56 and red or clear 58. The blue section 54 of the color filterhas the narrow pass spectral transmission characteristic 72 shown inFIG. 10. The green section 56 of the color filter has the narrow passspectral transmission characteristic 74 as shown in FIG. 10. The filter52 is positioned over the output ends of the fiber optic tubes so thatthe light supplied by blue light sources passes through the blue portion54 of the filter 52. Similarly, the light supplied by green lightsources passes through the green portion 56 of the filter 52. Lightsupplied by red light sources passes through the red (or clear) portion58 of the filter 52. In an embodiment of the invention constructed bythe inventors, the light supplied by the red light sources, can passthrough a substantially clear section of glass 58, i.e., the substratefor the blue 54 and green 56 filters because some photosensitivematerials have a very high tolerance to spectral variations in the redband.

The foregoing describes applicant's preferred method and apparatus forproducing high quality continuous tone and/or color images onphotosensitive material. It is recognized that numerous modificationsand/or variations will occur to those skilled in the art withoutdeparting from the spirit or scope of the invention.

1. Apparatus for producing a color composite image on a target surfaceof photosensitive material extending in perpendicular lateral andlongitudinal directions, said apparatus comprising: a source of digitalsignals describing said composite image; an imaging head comprised of aplurality of light sources including a first group of light sources anda second group of light sources; said first group including a pluralityof light sources respectively exhibiting spectral outputs within a firstspectral band; said second group including a plurality of light sourcesrespectively exhibiting spectral outputs within a second spectral band;said imaging head being mounted proximate to said target surface; acontroller responsive to said digital signals for selectively energizingsaid light sources to image substantially identically shaped pixels ontosaid target surface to form said composite image, said composite imagebeing formed of multiple longitudinally displaced pixel rows whereineach row is comprised of laterally displaced pixel images; a firstfilter mounted between the light sources of said first group and saidtarget surface, said first filter exhibiting a narrow pass spectraltransmission characteristic within said first spectral band for causingsaid photosensitive material to respond substantially uniformly to saidlight sources of said first group; and; a second filter mounted betweenthe light sources of said second group and said target surface, saidsecond filter exhibiting a narrow pass spectral transmissioncharacteristic within said second spectral band for causingphotosensitive material to respond substantially uniformly to said lightsources of said second group.
 2. The apparatus of claim 1 wherein saidimaging head includes a third group of light sources; and wherein saidfirst, second, and third groups of light sources respectively producelight within the green, blue, and red spectral bands.
 3. A method forproducing a color composite image on a target surface of photosensitivematerial, said method including the steps of: providing a plurality offirst light sources each energizable to produce light having acharacteristic spectral output where said first light sourcescollectively define a first spectral range; providing a plurality ofsecond light sources each energizable to produce light having acharacteristic spectral output where said second light sourcescollectively define a second spectral range; passing the light producedby said first light sources through a first filter having a spectraltransmission characteristic narrower than said first spectral range toconstrain the outputs of said first light sources to the spectralcharacteristic of said first filter; passing the light produced by saidsecond light sources through a second filter having a spectraltransmission characteristic narrower than said second spectral range toconstrain the outputs of said second light sources to the spectralcharacteristic of said second filter; applying energizing signals tosaid first and second light sources to produce a composite image; andimaging said composite image onto a target surface of photosensitivematerial.
 4. The method of claim 3 wherein each of said plurality offirst light sources produces light within a green spectral range andeach of said plurality second light sources produces light within a bluespectral range.